Learn about this topic in these articles:

association with Kramnik

...negotiations with FIDE, which recognized Kramnik as the “classical” world chess champion, he agreed to a unification match in 2006 with FIDE’s challenger, the Bulgarian grandmaster
Veselin Topalov, who had won the 2005 FIDE World Championship Tournament. Kramnik won the match. As part of the unification contract, the winner agreed to risk the consolidated title in FIDE’s 2007...

...chess federation), which recognized Kramnik as the “classical” world chess champion, he agreed to a unification match in 2006 with FIDE’s challenger, the Bulgarian grandmaster
Veselin Topalov, who had won the 2005 FIDE World Championship Tournament. Kramnik earned the match victory with five wins, seven draws, and four losses, with one of the losses by way of a forfeit...